Epistemology is the study of the nature, source, limits, and
validity of knowledge. It is especially interested in developing
criteria for evaluating claims people make that they "know" something.
In particular, it considers questions such as: What is knowledge?
What is the difference between knowledge and opinion or belief? If
you know something, does that mean that you are certain about it?
Is knowledge really possible?

Traditionally, philosophers have thought that if someone (P) knows X,
that means that

P believes that X is true

X is, in fact, true: we cannot know something that is false because
that which is false is not, and to know something that does not exist is
to know no thing (that is, to know nothing)

P can give a justification or rationale (the logos) for thinking
that X is true. Such a justification can be given by appealing to intuition
(immediate, personal certainty that X is true), reasoning (proving
that X is true based on shared strategies of argumentation), or sense
experience (public, repeatable, verifiable demonstration or experiment
showing that X is true).

Despite the fact that intuition is a common phenomenon, philosophers have
often been hesitant to identify it as a form of knowledge--primarily because
there seems to be little way to determine whether it does, in fact, provide
knowledge as opposed simply to lucky guesses. So most philosophers
focus, instead, on reason and sense experience as the bases of knowledge.
These two latter ways of approaching the question of knowledge are identified
as rationalism and empiricism.

A rationalist epistemology claims that knowledge (as opposed to opinion)
is possible only if it is based on self-evident and absolutely certain
principles. Such principles are not learned through experience; instead,
they are implicit in the very notion of reasoning (in Latin: ratio)
itself. Sense experience cannot provide the certainty needed to guarantee
that what we claim to know is true. So, like mathematicians, we have
to rely on reason itself as the basis for determining whether our opinions
are justified true beliefs (that is, knowledge).

Knowledge for the rationalist is what can be deduced from principles that undoubtable ("indubitable") or are true by definition. Examples of such principles include: "A whole is always greater than any one of
its parts," "A thing cannot be and not be at the same time in the same
respect," "Bachelors are unmarried males," "Unicorns are imaginary animals,"and
"Triangles have three sides." These statements are known with certainty
to be true because the very meaning of the terms involved (e.g., wholes,
parts, things, bachelors, unicorns, triangles) requires that some judgments
we make about them do not rely on sense experience. We thus know
about such things prior to any sense experience we have or could
have about them. This knowledge is called a priori.
Any knowledge that relies on (that is, comes after or is posterior
to) sense experience is called a posteriori.

Plato is an example of a rationalist. He says that sense experience
fails to provide us with any guarantee that what we experience is, in fact,
true. The information we get by relying on sense experience is constantly
changing and often unreliable. It can be corrected and evaluated for dependability
only by appealing to principles that themselves do not change. These unchanging
principles (or "Forms") are the bases of what it means to think or reason
in the first place. So if we can show that an opinion or belief we
have is based on these undoubtable principles of thought, we have a firm
foundation for the opinion. That foundation is what allows us to
think of a belief as more than simply opinion; it is what allows us to
identify the belief as justified and true, and that is what is meant by
knowledge.

In short, in order to have knowledge (justified true belief), we have
to transcend the ever-changing flux of the physical world and grasp
a permanent rational order behind the flux, an order that will demonstrate
the universal in the particular. This "grasping" is an intellectual
act of the mind, which, in its purest manifestation, is exclusively formal
(i.e., mathematical). Such an intellectual act can take place only
if there are certain innate ideas upon which it can be based. Knowing,
then, is an act of making the observable world intelligible by showing
how it is related to an eternal order of intelligible truths.

In other words, the world of changing, material objects (the visible
world) is merely a fleeting image of the intelligible world--what Plato
calls the realm of the Forms. Physical objects are real only insofar as
they are intelligible, but they can be intelligible only in terms of that
which does not change. What makes a thing intelligible as a certain kind
of thing cannot be constantly changing: otherwise, it could not be identified
as that kind of thing, nor would it be that kind of thing. So a thing is
what it is in virtue of something that is not changing. But since the visible
world is constantly changing, it cannot be used as the basis for identifying
what things are. There must be an intelligible (non-sensual) realm in terms
of which physical things are said to exist intelligibly. That is the realm
of the Forms.

Plato's simile of the sun, image of the divided line, and allegory of
the cave are intended to clarify exactly how the things we experience in
the sensible, ordinary world (e.g., chairs, drawn triangles) are less real
than the ideal models (Forms) on which they rely for their existence and
in terms of which they are intelligible. Just as drawings, reflections,
or copies of sensible objects are not as real as the sensible things on
which they depend, so sensible things are not as real as the concepts in
terms of which they are identifiable. Concepts that rely on sensual imagination
for their intelligibility--for example, mathematical concepts such as triangularity--are
more real than, say, triangular blocks of wood or drawings of triangles.
But even though concepts that are based on sense experience are not limited
to any particular expression and are unchanging, they are not as real as
the Forms, which do not rely for their existence or intelligibility on
anything sensual and changing.

Epistemology

Ontology

Source of Being and Intelligibility

Knowledge

Pure reason(grasped mathematically)

The Forms

The Intelligible World

The Good

Understanding (subsuming the particular under the general)

Concepts

Opinion (conjecture)

Belief, sense experience

Particular sensible objects

The Visible World

The Sun

Imagination, Hearsay

Images, Shadows, Reflections

Some Forms (e.g., chair-ness) are the ideal models in terms of which
physical objects (e.g., chairs) exist and are intelligible. Other even
higher Forms (e.g., equality, justice) provide the means by which not only
physical objects but also activities, relations, and even lower Forms themselves
are identifiable. The Forms are not abstractions or generalizations based
on our sensual experience of physical objects; rather, we know physical
objects as what they are by knowing them in terms of their Forms. As such,
in order to know that a chair is a chair, we have to know what chair-ness
is first, and that means that we cannot begin with sensible experience.
Likewise, in order to know that two numbers are equal, or that an action
is a just action, we have to know first what equality or justice is. But
that already assumes we know what a number or action is; and that can only
be known by appealing to lower Forms that rely for their intelligibility
and existence on higher Forms. The highest Forms are themselves intelligible
and exist ultimately in terms of the "super" Form, the Good.

Meno's Paradox and the Immortality of Soul: how will you know
what you are looking for if you first don't already know it (and thus have
no reason to go looking for it)? But why look for something you already
have? This is the paradox raised in Plato's dialogue called the Meno.
In answer to "Meno's Paradox," Plato suggests that before we were born
we existed in another realm of being (the realm of the Forms). The shock
of being born makes us forget what we knew in that realm. But when we are
asked the right questions or have certain experiences, we remember or "recollect"
innate (inborn) truths. So if we existed before our births, there is every
reason to think that we will continue to exist after our deaths.